Contact printing apparatus and method

ABSTRACT

Contact printing apparatus and method in which a high voltage electrostatic hold-down system is employed to electrostatically pin photosensitive film and a negative to a surface and to each other while an ionized laminar flow air stream is directed over the surface and the laminated sheets. An enclosure for the contact printing zone includes a thin, flexible, transparent plastic hood interposed between the light source and the film. The hood functions to shield the zone from falling dust and at the same time vibrates on the laminar flow air stream to nullify the effect of shadows produced by dust particles falling on the hood.

United States Patent 1191 Schweriner Oct. 29, 1974 [54] CONTACT PRINTING APPARATUS AND 3,723,001 3/1973 Zeunen et al 355/99 I METHOD [75] lnventor: Harold A. Schweriner, Hatboro, Pa. Pr'mary Exammer R lchurd F Attorney, Agent, or Firm-Stanley B1lker [73] Assignee: The Simco Company, lnc., Lansdale,

57 ABSTRACT [22] Flled: 1973 Contact printing apparatus and method in which a [21] Appl, No.1 342,965 high voltage electrostatic hold-down system is employed to electrostatically pin photosensitive film and a negative to a surface and to each other while an ion- ,[52] Cl ized laminar flow air stream is directed over the sur- 5] l t Cl 6 2/7/02 face and the laminated sheets. An enclosure for the l H3 12 contact printing zone includes a thin, flexible, trans- [5 l 0 "2 i 1 parent plastic hood interposed between the light l l source and the film. The hood functions to shield the 56 R f Ct d zone from falling dust and at the same time vibrates l l e erences e on the laminar flow air stream to nullify the effect of UNITED STATES PATENTS shadows produced by dust particles falling on the 3,128,492 4/1964 Hanscom et al. l5/l.5 X hOOCl. 3,480,364 ll/l969 Barnett et al...1.. 355/[13 X I 3,578,862 5/1971 l-ludock et al. 355/117 x 7 Claims, 3 Drawmg Figures e) e (a 6 (+9 9 6 6) 69 6 9 30 l- 1 I I- I I -T- 26 -T- l 1 i I 1- T- A l 1 1 I I 1 i /4--7 7 4; /5- "a \:\z aw/ CONTACT PRINTING APPARATUS AND METHOD In the conventional contact printing apparatus, one or more sheets of a negative and photosensitive film are placed upon the surface of an opaque plate preparatory to exposing the laminate to a light source. During the contact printing procedure, it is important that the film and the photosensitive sheet be in intimate face-to-face contact while at the same time provisions must be available to permit manipulation of the various components and elements with respect to each other in order to assure appropriate registration of a composite arrangement. It is also important to prevent attraction and entrapment of dust particles upon the interfacial areas since these would tend to detract from the quality of the finished print. ln addition, it is significant that there be no undue and uncontrolled attracting or repelling of the various sheets with respect to each other as a result of the generation of static electricity effected by rubbing of the sheets together.

In the past, electrostatic hold-down and clamping systems have been utilized as a convenient means for pinning the sheets to a printing surface and for laminating the sheets together. However, one of the objections to the heretofore employed electrostatic pinning systems has been the resultant tendency for the electrostatically charged surfaces to react with electrically charged dust particles so as to cause deposition of these particles upon photosensitive surfaces.

The present contact printing apparatus utilizes the principles of directing a stream of filtered air in laminar flow disposition over the surfaces which are to be electrostatically pinned. At the same time, this ultra-cleam laminar flow of air is subjected to ionization by means of a high voltage A.C. static eliminator whereby ions of both polarities are blown over the surfaces of the printing table and the sheets which are to be laminated. The high side of a unidirectional high voltage generator is applied to the undersurface of the table-top to which the sheets are to be secured such that the various sheets may be polarized when placed upon the table-top. A grounding element is applied to each sheet as it is laid upon the surface of the printing table or the underlying sheet whereby a conductive path is provided back to the other side of the high voltage from the sheet contacted thus enabling charges to be transported along such path. In this manner, opposite charges are produced at the opposing interfaces so as to cause such interfaces to be electrostatically adhered to each other. At the same time, the continuous stream of ionized air in laminar flow paths is directed over the sheets to maintain a continuous path to ground as well as to sweep the area clean.

An enclosure for the contact printing zone includes a thin, flexible, transparent plastic hood which is interposed between the light source and the electrostatically adhered sheets. The hood functions to shield the zone from falling dust and in addition is caused to vibrate by the laminar flow stream therebelow so as to nullify the effect of shadows produced by dust particles falling upon the hood.

After the contact printing is completed, by pulling the sheets apart in the filtered ionized air stream, the sheets are then rendered neutral, thus eliminating any tendency to be attracted or repelled with respect to any adjacent charged parts or to prevent further attraction of dust. Furthermore, neutralization of the sheets'being separated in the ionized air stream eliminates any electrostatic sparking which might otherwise occur during stripping or in touching the sheets togrounded parts thereby avoiding exposure of the photosensitive surfaces to sparks that would interfere with image quality.

It is therefore an object of this invention to provide a contact printing apparatus and method in which the various sheets may be electrostatically pinned to each other and to a surface in a substantially dust-free environment.

Another object of this invention is to provide a contact printing apparatus and method in which an ultra-clean laminar flow air system is employed to furnish a dual polarity ionized background.

Yet another object of this invention is to provide a contact printing apparatus and method in which the lamination, the printing operation, and the removal of the sheets from laminated disposition are all accomplished in a dust-free neutralizing atmosphere.

A further object of this invention is to provide a contact printing apparatus in which static sparks are entirely avoided so as to prevent accidental exposure of photosensitive surfaces which would interfere with image quality.

A still further object of this invention is to provide a contact printing apparatus in which shadows caused by dust particles are essentially eliminated.

Other objects of this invention are to provide an improved device and method of the character described which is easily and economically produced, sturdy in construction, and highly efficient and effective in operation.

With the above and related objects in view, this invention consists of the details of construction and combination of parts as will be more fully understood from the following detailed description when read in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view, and partly in section, of a contact printing apparatus embodying this invention.

FIG. 2 is a sectional view demonstrating the first step in the practice of this invention and in the charge distribution resulting therefrom.

H6. 3 is a sectional view demonstrating a second step in the practice of this invention and thecharge distribution of the various elements.

Referring now in greater detail to the drawings in which similar reference characters refer to similar parts, there is shown a contact printing apparatus comprising an electrostatic, printing frame, generally designated as A, and an ionized ultraclean laminar flow air system, generally designated as B, for directing a filtered air stream having ions of both polarities overthe zone above the printing frame.

The printing frame A essentially constitutes a flat base plate 12 made of a dielectric insulating material, such as a phenolic resin, to the lower surface of which is applied a conductive element 14, such as a film or layer of copper, in intimate contact with a substantial portion thereof. An insulative sheet 16 of pressed wood or chipboard sandwiches the conductive layer 14 against the base plate 12. A high voltage DC. power supply 18 is connected to the conductive element 14. The power supply 18 is a conventional unidirectional electrostatic power generator which can develop a DC potential in the range of 1,000 to 15,000 volts with respect to ground. The generator 18 is shown in block or schematic form, and for the purpose of the instant invention, develops a negative output when switch 20 (located on the control panel) is closed. A voltage control 22 is also located on the instrument panel and permits the voltage output to be varied as desired within the range. A light source 25 is suspended above the base plate 12 in order to direct rays of light for exposing the laminate, for example, a photosensitive sheet 26 having an overlying film negative 28 thereon. A grounding member 30, such as a roller or brush, which is connected by way of a lead 32 to ground (i.e., the other side of the DC. high voltage power supply 18). I

The laminar flow ionization system B comprises a chamber 34 having a grilled inlet opening 36 covered by a pre-filter 38 and a multi-apertured outlet 40 covered by high efficiency particulate filter 42 on the downstream side. A fan or blower 44 draws air through the inlet 36 and filter 38 and then forces the air toward the lower portion of the chamber 34 where a static head is built up againstthe face of the high efficiency filter 42. In this manner, the chamber 34 acts as a plenum to squeeze the air through the filter 42 and thence through the apertures 40 in a series of parallel laminar flow paths.

The filter 42 is a high efficiency particulate filter which only passes submicron particles, such as is made by Cambridge Filter Company for ultraclean air installations. By circulating a large volume of the submicron particle filtered air in laminar flow disposition across the working area, the ultra-clean air stream sweeps the zone above the plate 12 of any dust particles.

An area static eliminator 45 is arranged in a network about the apertured outlet 40. The area static eliminator constitutes a series of conductive points 46 which are embedded in an insulatively coated conductive cable 48 formed in a sinuous path over the face of the outlet 40. The points 46 are axially disposed with respect to the laminar air flow in adjacently spaced disposition with respect to a grid 50 of grounded conductive rods. An A.C. high voltage 52 is connected to. the points 46 through the cable conductor 48 and across the grid 50 whereby ions are created in the gap therebetween. Accordingly, a plurality of positive and negative ions are formed in the laminar air flow path which is swept over the zone above the printing frame A.

In order to secure the sheets 26 and 28 to the face of the printing frame A and to each other, each sheet is first exposed to the ionized laminar air stream which effectively neutralizes the sheets and cleans them of any dust particles. Sheet 26 is first laid upon the surface of insulative plate 12 and the grounding element is moved across the upper surface of the sheet 26 thus causing polarization, as shown in FIG. 2. That is, the negative voltage applied to the layer 14 causes the adjacent surface of the plate 12 to be charged positively while the distal surface of the plate 12 becomes negatively charged. Correspondingly, the surface of sheet 26 in contact with plate 12 becomes positively charged whereas the upper surface of the sheet 26 is polarized negatively. Thus, the sheet 26 becomes electrostatically pinned to the upper surface of the plate 12 by virtue of the opposed charges at opposite sides of the interface. After removal of the grounding roller 30, the

stream of positive and negative ions arriving in the laminar flow of air continues to act as a conductive background which serves as a supply of ions which maintains the polarizing effect while at the same time retaining the zone thereabove neutral and sust-free.

Next, by laying negative sheet 28 upon the surface of the sheet 26 and drawing the grounding element 30 I across the upper surface of sheet 28, polarization of the laminate is effected as shown in FlGf3. Accordingly, the sheets of the laminate become electrostatically pinned to each other and are adhered to the plate 12. At this stage, the contact printing may be accomplished by directing the rays from lamp 25 upon the sheet 28 whereby the photosensitive surface of sheet 26 is exposed through the clear areas thereof. Note that the electrostatic pinning procedure allows the various sheets to be manipulated laterally with respect to each other by lifting and then replacing in registration of the composite elements.

An enclosure formed about the contact printing zone acts in conjunction with the laminar air flow system as a barrier to keep dust particles out of the area and includes a back wall 54 and a hood 56. The hood 56 is of thin, flexible, transparent plastic, such as Tedlar resin, which permits passage of light waves between ultra-violet and infra-red bands, thereby allowing the full range of light to which the film sheets are photosensitive to be passed by the light source 25 through the hood. The hood 56 functions to shield the contact printing zone from falling dust and at the same time vibrates on the laminar flow air stream to nullify the ef fect of shadows which would be produced by dust particles falling on the hood.

After contact printing, the laminated sheets 26 and 28 may be lifted together from the face of the table-top 12. However, these sheets may adhere together by virtue of the trapped opposing charges at their contacting interfaces. Thus, even though the laminate may be effectively neutralized by the neutralizing atmosphere of the ionized laminar flow air stream, the opposed charges at the interface remain relatively tightly bound. By peeling the sheets 26 and 28 from each other while both are subjected to the ionized air stream bath, each individual sheet becomes entirely neutralized so as to resist attraction of any dust with respect thereto. In addition, since each sheet 26 and 28 is now electrically neutral, uncontrolled attractive and/or repelling forces are essentially eliminated with respect to adjacent members. Finally, by pulling the sheets apart in the neutralizing stream, electrostatic sparking which might otherwise occur during stripping of the sheets apart is avoided thereby preventing exposure to spurious light points which would interfere with image quality. Although this invention has been described in considerable detail, such description is intended as being illustrative rather than limiting since the invention may be variously embodied without departing from the spirit thereof, and the scope of the invention is to be determined as claimed.

What is claimed is:

1. Contact printing apparatus comprising:

a surface defining a zone for contact printing,

means for securing a photosensitive sheet upon said surface and a negative in face-to-face abutment with said photosensitive sheet in contact with said surface,

means for directing an ionized filtered stream of air in laminar flow disposition over said contact printing zone,

a light source interposed above said contact printing zone, and,

an enclosure at least partially overlying the contact printing zone, said enclosure including a hood of thin, flexible transparent material interposed between the light source and the sheets to be exposed, said hood operating to shield the contact printing zone from falling dust and at the same time vibrating on the laminar flow air stream to nullify the effects of shadows produced by dust particles falling on the hood.

2. A method for contact printing comprising the steps a. directing a stream of ultra-clean, filtered air in laminar flow disposition over a surface defining a contact printing zone while effecting D.C. charging of the surface,

b. electrostatically ionizing the filtered laminar flow air stream to generate ions of positive and negative polarities within said air stream, and while steps (a) and (b) are being performed,

c. passing a first sheet through the filtered and ionized laminar flow air stream,

d. electrostatically pinning the first sheet against the surface,

e. passing a second sheet through the filtered and ionized laminar flow air stream,

f. electrostatically pinning the second sheet to the first sheet while the latter is in electrostatically pinned disposition with the surface,

g. directing rays of light through one of said sheets to expose the other of said sheets, and

h. separating said sheets from the surface and from each other,

whereby (i) the sheets are cleansed and maintained in dust-free condition during pinning, (ii) the ionized laminar flow air stream acts to support the electrostatic holding forces maintaining the sheets in pinned disposition during exposure, and (iii) the sheets can be separated from each other in a staticfree environment.

3. A contact printing apparatus for electrostatically pinning a photosensitive sheet and at least one negative to a surface and in face-to-face disposition with each other preparatory to exposing said photosensitive sheet to light rays through each said negative comprising:

an insulative plate,

means for directing a stream of filtered, ultra-clean air in laminar flow disposition over one surface of said insulative plate in a predetermined direction substantially parallel to said surface so as to sweep the zone above said insulative plate with unidirectional ultra-clean air,

electrostatic discharge means for generating ions of both polarities within the laminar flow air stream so that both positive and negative ions permeate the ultra-clean air zone,

means for applying with respect to ground a unidirectional high voltage to the opposite surface of said insulative plate,

applicator means coupled to ground for contacting the outermost surface of said photosensitive sheet and each said negative as they are respectively placed in successive abutment with the first surface of said insulative plate and with each other, and

means for directing light rays through each said negative to expose the photosensitive sheet in pinned disposition therewith,

whereby the ionized laminar flow air stream (l cleanses the photosensitive sheet and each negative prior to pinning, (2) acts to support the electrostatic holding forces maintaining the photosensitive sheet and each negative in pinned disposition during exposure to the light rays, and (3) provides a static-free environment during separation of the photosensitive sheet from each said negative.

4. the apparatus of claim 3 wherein said electrostatic discharge means comprises an area static eliminator.

5. The apparatus of claim 4 wherein said area static eliminator emits ions along a path co-directional with the laminar flow stream.

6. The apparatus of claim 3 wherein a light source is interposed above the contact printing zone.

means for directing a filtered stream of ultra-clean ionized air in laminar flow disposition unidirection ally over the first surface,

electrostatic means for pinning a photosensitive sheet and at least one negative sheet upon said first surface and in electrostatically pinned disposition with each other, said electrostatic means including a unidirectional high voltage having one side coupled to the second surface of said insulative plate and means constituting a conductive path from the other side of said unidirectional high voltage to an outboard surface of a sheet whose other face is in pinned disposition with the first surface of said insulative plate, and

' a light source for exposing the photosensitive sheet through each said negative sheet, and

means for generating ions of both polarities within the laminar flow air stream so that both positive and negative ions permeate the ultra-clean air zone and reinforce electrostatic pinning forces. 

1. Contact printing apparatus comprising: a surface defining a zone for contact printing, means for securing a photosensitive sheet upon said surface and a negative in face-to-face abutment with said photosensitive sheet in contact with said surface, means for directing an ionized filtered stream of air in laminar flow disposition over said contact printing zone, a light source interposed above said contact printing zone, and, an enclosure at least partially overlying the contact printing zone, said enclosure including a hood of thin, flexible transparent material interposed between the light source and the sheets to be exposed, said hood operating to shield the contact printing zone from falling dust and at the same time vibrating on the laminar flow air stream to nullify the effects of shadows produced by dust particles falling on the hood.
 2. A method for contact printing comprising the steps of: a. directing a stream of ultra-clean, filtered air in laminar flow disposition over a surface defining a contact printing zone while effecting D.C. charging of thE surface, b. electrostatically ionizing the filtered laminar flow air stream to generate ions of positive and negative polarities within said air stream, and while steps (a) and (b) are being performed, c. passing a first sheet through the filtered and ionized laminar flow air stream, d. electrostatically pinning the first sheet against the surface, e. passing a second sheet through the filtered and ionized laminar flow air stream, f. electrostatically pinning the second sheet to the first sheet while the latter is in electrostatically pinned disposition with the surface, g. directing rays of light through one of said sheets to expose the other of said sheets, and h. separating said sheets from the surface and from each other, whereby (i) the sheets are cleansed and maintained in dust-free condition during pinning, (ii) the ionized laminar flow air stream acts to support the electrostatic holding forces maintaining the sheets in pinned disposition during exposure, and (iii) the sheets can be separated from each other in a static-free environment.
 3. A contact printing apparatus for electrostatically pinning a photosensitive sheet and at least one negative to a surface and in face-to-face disposition with each other preparatory to exposing said photosensitive sheet to light rays through each said negative comprising: an insulative plate, means for directing a stream of filtered, ultra-clean air in laminar flow disposition over one surface of said insulative plate in a predetermined direction substantially parallel to said surface so as to sweep the zone above said insulative plate with unidirectional ultra-clean air, electrostatic discharge means for generating ions of both polarities within the laminar flow air stream so that both positive and negative ions permeate the ultra-clean air zone, means for applying with respect to ground a unidirectional high voltage to the opposite surface of said insulative plate, applicator means coupled to ground for contacting the outermost surface of said photosensitive sheet and each said negative as they are respectively placed in successive abutment with the first surface of said insulative plate and with each other, and means for directing light rays through each said negative to expose the photosensitive sheet in pinned disposition therewith, whereby the ionized laminar flow air stream (1) cleanses the photosensitive sheet and each negative prior to pinning, (2) acts to support the electrostatic holding forces maintaining the photosensitive sheet and each negative in pinned disposition during exposure to the light rays, and (3) provides a static-free environment during separation of the photosensitive sheet from each said negative.
 4. the apparatus of claim 3 wherein said electrostatic discharge means comprises an area static eliminator.
 5. The apparatus of claim 4 wherein said area static eliminator emits ions along a path co-directional with the laminar flow stream.
 6. The apparatus of claim 3 wherein a light source is interposed above the contact printing zone.
 7. Apparatus for contact printing comprising: an insulative plate having first and second surfaces, means for directing a filtered stream of ultra-clean ionized air in laminar flow disposition unidirectionally over the first surface, electrostatic means for pinning a photosensitive sheet and at least one negative sheet upon said first surface and in electrostatically pinned disposition with each other, said electrostatic means including a unidirectional high voltage having one side coupled to the second surface of said insulative plate and means constituting a conductive path from the other side of said unidirectional high voltage to an outboard surface of a sheet whose other face is in pinned disposition with the first surface of said insulative plate, and a light source for exposing the photosensitive sheet through each said negative sheet, aNd means for generating ions of both polarities within the laminar flow air stream so that both positive and negative ions permeate the ultra-clean air zone and reinforce electrostatic pinning forces. 